620 VDC [RE-wrenches]

[Bill Brooks]

${top_text_ad}
All,

The key is coming up with a defensible number for "lowest expected ambient
temperature." The NEC does not define this number. I have submitted a
proposal to our Technical Review Committee to use ASHRAE weather data for
this number. My recommendation is as follows:


690.7(A)
FPN: One source for statistically valid lowest expected ambient temperature
design data for various locations is the Extreme Annual Mean Minimum Design
Dry Bulb Temperature found in the ASHRAE Handbook — Fundamentals. These
temperature data should be used to calculate maximum voltage using either
manufacturer’s temperature coefficients relative to the rating temperature
of 25°C, or Table 690.7.



This information will be produced in a publically-available design document
so that anyone can access the data. The ASHRAE number is significantly
warmer than the record cold. As Drake pointed out, even a 480Vac system has
maximum voltage of about 680Vac. The modules, wire, and disconnects are not
the issue as much as the inverter. Many inverters have been built to handle
voltages far in excess of their maximum (SatCon was actually listed to
660Vdc), but others are not. Most inverters will merely wait while the array
warms up to start rather than spontaneously combusting at 601.

The question is what is the statistically likelihood of cold temperatures
and high irradiance on the system. The code allows for short-term excesses
on various current and voltage parameters. The key is if we are able to
convince the code making panel that we have reduced the likelihood of a
problem sufficiently.

Bill.


-----Original Message-----
From: Bob-O Schultze, Electron Connection
[mailto:bob-o at electronconnection.com] 
Sent: Thursday, June 26, 2008 1:59 PM
To: RE-wrenches at topica.com
Subject: Re: 620 VDC [RE-wrenches]


Drake,
I'd have to agree with Walt that you are up against the Code anyway  
you look at it.
That said, let's look at reality:
We're talking Voc here- no current of any kind flowing.
We're talking Voc at the record lowest temperature recorded.
Clearly, 600V wire does not break down at 601V

Does anyone seriously believe this is -or ever could be in the real  
world- a safety issue?
Not me.
Cheers, bob-O


On Jun 26, 2008, at 12:37 PM, Drake Chamberlin wrote:

>
> Hello Wrenches,
>
> Some inverters are able to withstand voltages above 600 VDC.  The  
> Solectria 95KW can take up to 620 VDC without harming the electronics.
>
> If we size a system so that the record low temperature is gives a  
> voltage of under 620 VDC, a viable system can be installed.  There  
> is a question, however, about the Code compliance of this, since  
> building wire is rated at 600 volts.  Higher voltage wiring is  
> available, but will likely not be cheap or convenient to use.   
> Also, what about 600 V modules and PV cable?
>
> Since this is DC, there are not any cycles to create a voltage  
> peak.  A 600 VAC RMS nominal system can have (if my calculations  
> are correct) a peak voltage of close to 850 volts.  Also, system  
> voltages can range up to 10% from the nominal, resulting in  
> voltages of as high as 660 VAC RMS on 600 volt systems.
>
> The question is, "can we have a system with a possible low- 
> temperature, open circuit voltage of 620 VDC with wiring and  
> modules rated for 600 volts?  Can the other 20 volts be considered  
> within the nominal range?
>
> Thanks,
>
> Drake


- - - -
Hosted by Home Power magazine

To send a message: RE-wrenches at topica.com

Archive of previous messages: http://lists.topica.com/lists/RE-wrenches/read

List rules & how to change your email address:
www.mrsharkey.com/wrenches/etiquette.php

Check out participant bios: www.mrsharkey.com/wrenches/

Moderator: michael.welch at homepower.com

${bottom_text_ad}
- - - -
Hosted by Home Power magazine

To send a message: RE-wrenches at topica.com

Archive of previous messages: http://lists.topica.com/lists/RE-wrenches/read

List rules & how to change your email address: www.mrsharkey.com/wrenches/etiquette.php

Check out participant bios: www.mrsharkey.com/wrenches/

Moderator: michael.welch at homepower.com
--^----------------------------------------------------------------
This email was sent to: ${recipient}

EASY UNSUBSCRIBE click here: http://topica.com/u/?bz8Qcs.${encoded_sub_id}.${EMC}
Or send an email to: RE-wrenches-unsubscribe at topica.com

For Topica's complete suite of email marketing solutions visit:
http://www.topica.com/?pXFOOTER
--^----------------------------------------------------------------